Producing a radar function for sensing airborne obstacles that are not cooperative for aircraft, in particular for drones, is essential to enable autopiloted aircraft to be included in the non-segregated air domain. It participates in the obstacle sensing and avoidance function called “Sense and Avoid”.
The field of application of the invention is notably that of short- and medium-range radars, which do not require a large antenna surface area, but do require a very good angular accuracy. Such is the case in particular with radars intended for the “Sense & Avoid” function.
The range of a radar is proportional, notably, to the surface area of its receiving antenna. This applies regardless of how the space is operated, this mode of operation of the space possibly being based on mechanical scanning, sequential electronic scanning or even beam forming by the FFC calculation, provided, however, that it is possible to swap spatial gain in transmission for coherent gain according to time in reception.
Moreover, the angular accuracy of a radar is proportional, in a first approach to the ratio λ/H in which λ is the wavelength and H is the length of the antenna in the plane in which the angular measurement is to be made. High accuracy may require a great length H. However, there is a problem notably due to the fact that this length may prove pointless from the range point of view if the antenna is not a lacuna antenna.
The above reasoning applied for one dimension is easily extended to two dimensions, on two axes, for example the azimuth and the elevation.
Currently, this problem is notably resolved by a very accurate angular measurement, but one which is very greatly ambiguous, obtained with an interferometer in reception, comprising two antennas, for which the distance between phase centres is significant.
To eliminate the ambiguities, one known technique entails using an interferometer with a number of antennas whose phase centres are irregularly spaced. In this case, at least three antennas are necessary.
However, this quickly leads to an implementation that is complex and of large size if the elimination of the angular ambiguities is to be reliable.